1. Field of the Invention
This invention relates generally to wafer bonding, materials integration, and heterogeneous integration.
2. Description of Prior Art
Wafer bonding refers to the use of pressure, heat, and in some cases interfacial adhesive layers to combine dissimilar materials in one monolithic semiconductor structure. Most wafer bonding involves the integration of two dissimilar substrates, such as Gallium Arsenide (GaAs) and Indium Phosphide (InP) in the vertical direction [J. Dudley, “Wafer Fused Vertical Cavity Lasers,” Ph.D. Dissertation University of California, chapter 4, 1994.]. FIG. 1 illustrates this prior art, where an InP substrate 30 with a grown epitaxial region 33 is bonded to a GaAs substrate 31 with a grown epitaxial region 34, at a wafer bonded interface 32. After removal of either the GaAs substrate 31 or the InP substrate 30 in FIG. 1, the combined epitaxial structure rests on one substrate. The technique of FIG. 1 enables integration of dissimilar materials in the vertical direction only, and not in the lateral direction.
A number of techniques have been demonstrated to integrate dissimilar materials in the lateral direction. These fall into two categories: epitaxial growth techniques (FIGS. 4–5) and wafer bonding techniques (FIGS. 2–3). FIGS. 4A–C illustrate the technique of epitaxial regrowth. Referring to FIG. 4A, a first epitaxial region 71 is grown on a substrate 70. The region 71 is etched away over a portion of the wafer 70, as shown in FIG. 4B. Finally, FIG. 4C shows a region 72 re-grown in the etched region, creating regions 71 and 72 adjacent to each other on the common substrate 70. The technique of FIGS. 4A–C is limited because the regrown region 72 must have the same lattice structure as the substrate 70 and the first epitaxial region 71. In addition, the regrown material 72 is of inferior optical quality relative to the as-grown material 71. For this reason, regrown material is generally optically passive or incapable of producing optical gain. Even optically passive regions, such as tuning regions in a tunable laser, suffer from non-radiative charge re-combination at regrown interfaces, leading to reduced tuning efficiency.
FIG. 5 illustrates another prior art epitaxial growth technique referred to as selective area growth. In this technique, a semiconductor wafer 80 is coated with a patterned silicon dioxide coating 81. Epitaxial material only nucleates or grows where the silicon dioxide is etched away, resulting in an epitaxial layer 82. FIG. 6 illustrates that the epitaxial layer 82 grows fastest where the silicon dioxide window is narrowest. Using a variable width window allows the growth rate at different parts of the wafer to be different. This technique can be used to fabricate semiconductor quantum wells of differing thickness and differing resultant emission wavelengths, but cannot be used to create large composition variation across a wafer.
FIGS. 2–3 illustrate prior art wafer bonding approaches to achieving epitaxial variation in the lateral direction. FIG. 2 illustrates the technique of aligned wafer bonding [Elias Towe, ed. “Heterogeneous Opto-Electronic Integration,” SPIE Press, 2000, Bellingham, Wash., Chapter 1.], where a first substrate 51 with a partially etched first epitaxial structure 52 is bonded to a second substrate 50 with a second epitaxial structure 53 that has been etched in a fashion complementary to the etch of epitaxial structure 52. This technique allows integration of largely dissimilar regions, but is limited to 2 regions, and there remains a gap 54 that is at least 1–2 microns wide (established by the alignment precision of the aligned bonding technique) between the two regions in the final structure. This eliminates the possibility of low-loss optical connectivity between the regions.
FIGS. 3A–C show a technique using non-planar wafer bonding [J. Geske, Y. L. Okono, J. E. Bowers, and V. Jayaraman, “Vertical and Lateral Heterogeneous Integration,” Applied Physics Letters, vol. 79, no. 12, pp. 1760–1762]. Referring to FIG. 3A, first, second and third epitaxial regions 61,62, and 63 respectively are grown vertically adjacent on a source substrate 60. Each of the regions 61–63 is partially etched to reveal the surfaces of all regions. The regions are then butted against a host substrate 64, as shown in FIG. 3B. The source substrate 60 and epitaxial regions 61–63 deform to make contact with the host substrate 64. FIG. 3C shows the structure that remains after the source substrate 60 and portions of the regions 61–63 have been etched away. What remains is the 3 epitaxial regions 61,62 and 63 integrated side by side on the host substrate 64, with gaps between them where deformation regions have been etched away. In this way, vertical integration is converted to lateral integration. Although this technique enables the integration of a large number of epitaxial regions, the necessity of gaps between the regions eliminates the possibility of low-loss optical connectivity between them. In addition, because the regions are integrated vertically prior to bonding, they must all be of the same lattice structure and dimension. Furthermore, strain limits on the combined vertical structure limit the allowable strain per epitaxial region, and deformation limits on the source wafer limit the thickness of each epitaxial region.
In summary, prior art epitaxial growth techniques of lateral integration require lattice compatibility between the regions, lead to compromised optical quality in the case of re-growth, and enable only lateral thickness variation in the case of selective growth. Prior art wafer bonding techniques suffer from one or more of the following problems: a small number of allowable lateral regions, constraints on region composition and thickness, and gaps which eliminate the possibility of low loss optical connectivity between regions.
From the foregoing, it is clear that what is required is an integration technique that allows the lateral integration of a large number of epitaxial regions with excellent optical quality of each region, low-loss optical connectivity between regions, and a minimum of constraints on the composition of each region.